1. Field of the Invention
This invention relates to display systems in general and more particularly, to a technique for packing and controlling a RAM refresh buffer for efficient utilization of the buffer both as storage of characters to be displayed as well as a source of the beginning point of each line of data to be displayed.
2. Description of the Prior Art
Numerous patents have addressed the general control of a refresh buffer to achieve efficient utilization of it in a display system.
U.S. Pat. No. 3,683,359 to Kleinschnitz, entitled "Video Display Terminal with Automatic Paging", filed Apr. 30, 1971, issued Aug. 8, 1972, is a display system which has a number of advantages over alternate types of systems. It utilizes a large memory and consequently can hold a large amount of data which can be displayed at one time. It provides a means for changing the portion of the memory to be displayed without destroying old data, and when used with a line end code, variable length lines can be stored using equivalent variable length memory slots. In addition, vertical scrolling in this system is simple. It, however, does have several disadvantages in that all of the lines displayed must be stored in sequence in the memory. That is, no alteration of line format can be accomplished without rewriting the memory. Additionally, there is a relatively large amount of logic hardware required and finally, any special data (menus, etc.) requires a dedicated sequential block of memory.
U.S. Pat. No. 4,117,469 to Levine, entitled "Computer Assisted Display Processor Having Memory Sharing by the Computer and Processor", filed Dec. 20, 1976, issued Sept. 26, 1978, is directed toward a display system which includes all of the advantages discussed in connection with the '359 patent. In addition, it has advantages such as any line(s) in memory can be displayed in any order desired without rewriting the memory. Also, new lines can be added and inserted anywhere on the display and the existing lines shifted in position without rewriting. Functions such as horizontal scrolling, line inserts and deletes can be done easily. Finally, multiple special data (menus) screens can be assembled from individual lines, i.e., common information can be displayed from the same memory slot for two or more memories. This patent does, however, have several disadvantages. First, it requires a dedicated microprocessor to handle the loading of the address counter in addition to the system processors time used to update data. Additionally, if the processor cannot respond in time one entire horizontal scan will be blank showing up as flicker on the CRT.
U.S. Pat. No. 4,129,858 to Toshitaka Hara, entitled "Partitioned Display Control System", filed Mar. 23, 1977, issued Dec. 12, 1978, contains several advantages of the ;359 patent and in addition, it requires minimal additional logic to provide several desirable features. However, it does have several disadvantages. That is, all the lines displayed must be in sequence in the memory and any special data (menus) requires a dedicated sequential block of memory. In addition, it can only select between screens of data and memory must be reserved for each position of every line whether used or not. Thus, the memory cannot be packed in this system.
U.S. Pat. No. 3,827,041 to Cook, entitled "Display Apparatus with Visual Segment Indicia", filed Aug. 14, 1973, issued July 30, 1974, has the advantage that it provides a large memory which holds more information than can be displayed at one time and there is provided means for changing the portion of the memory to be displayed without destroying old data. Again, however, it has the disadvantage that all lines displayed must be in sequence in the memory; large amounts of logic hardware are required; any special data (menu) requires a dedicated sequential block of memory; and there must be memory reserved for each position of every line whether the positions are used or not which results in inefficient use of the memory.